Conventional crystal growth techniques involve processing supercritical fluids in a high-pressure apparatus. Such an apparatus is disclosed, for example, in D'Evelyn et al., U.S. Pat. No. 8,435,347 (incorporated herein by reference). FIG. 1 is a simplified diagram illustrating a high pressure apparatus 100 according to an embodiment of D'Evelyn et al. The apparatus comprises a stack of two or more radial restraint structures 100, comprising a high strength enclosure ring 101 and a ceramic ring 103. The stack surrounds heater 105, and the interior of heater 105 defines a processing chamber, into which capsule 107 may be placed. Thus, the heater is positioned between the capsule and one or more radial restraint structures. The stack provides radial confinement for pressure generated within capsule 107 and transmitted outward through heater 105. Axial confinement of pressure generated within capsule 107 may be provided by end plugs 111, crown members 117, and tie rods or tie rod fasteners 115. Apparatus 100 may also include a pressure transmission medium 109 proximate to the axial ends of capsule 107 and to end plugs 111 according to a specific embodiment.
Although this apparatus design represents a significant improvement over conventional apparatus configurations, Applicants have discovered that during high pressure reactions, the apparatus can distort. Specifically, Applicants have discovered that under very high pressure, the heater within the interior region defined by die stack may extrude into gaps between adjacent ring assemblies of the die stack, causing cracking of the corners of rings assemblies and making it more difficult to remove the heater from the ring assemblies for maintenance or replacement. The Applicants have further discovered that this undesirable extrusion and damage may be prevented by axially restraining the ring assemblies in the die stack.
Therefore, there is a need for an improved apparatus configuration that can withstand extremely high process pressures. The present invention fulfills this need among others.